Failure-to-file penalty

If you owe tax and file your return after the deadline (including extensions), the IRS charges 5% of the unpaid tax for each month or part of a month the return is late, up to 25%. It's ten times the failure-to-pay rate, so filing on time, or getting an extension, matters even if you can't pay in full yet. An extension gives you more time to file, but not more time to pay.